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What Is Power Ultrasonic and Its Applications

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What Is Power Ultrasonic and Its Applications What is Power ultrasonic and its applications. • Sound is the propagation of smallest pressure and density variations in an elastic medium (gas, liquid, solid-state body). For example, a noise is generated when the air in a specific spot is compressed more than in the surrounding area. Subsequently, the layer with changed pressure propagates remarkably fast in all directions at speed of sound of 343 m/s. Acoustic frequencies between 16 (15) kHz and 1 GHz are referred to as ultrasound; in industrial settings we call it “ultrasonics”. To clarify: people are able to hear frequencies between 16 Hz and 20 kHz; i.e., the lower frequencies of industrial ultrasonics are audible, especially if secondary frequencies are generated since ultrasonic is in 1/3 octaves, the first sound you hear is 2/3 octave namely around 13kHz (this gives the high pitching sound one often connects with ultrasonic). And what is more, the ultrasonics can be felt when touching the weld tool. For ultrasonic welding, the frequency range is between 15 kHz and 80 kHz. Additional fields of application: Imaging ultrasound in the field of medical diagnostics ranges between 1 and 40 MHz It is not audible or cannot be felt. In the field of industrial material testing, ultrasonics is used at frequencies from 0.25 to 10 MHz • Classification of ultrasonic frequency ranges – the range audible to the human ear only makes up a fraction of the entire span The Theory of Sound Waves In order to understand the mechanics of ultrasonics, it is necessary to first have a basic understanding of sound waves, how they are generated and how they travel through a conducting medium. The dictionary defines sound as the transmission of vibration through an elastic medium which may be a solid, liquid, or a gas. Sound Wave Generation - A sound wave is produced when a solitary or repeating displacement is generated in a sound conducting medium, such as by a "shock" event or "vibratory" movement. The displacement of air by the cone of a radio speaker is a good example of "vibratory" sound waves generated by mechanical movement. As the speaker cone moves back and forth, the air in front of the cone is alternately compressed and rarefied to produce sound waves, which travel through the air until they are finally dissipated. We are probably most familiar with sound waves generated by alternating mechanical motion. There are also sound waves which are created by a single "shock" event. An example is thunder which is generated as air instantaneously changes volume as a result of an electrical discharge (lightning). Another example of a shock event might be the sound created as a wooden board falls with its face against a cement floor. Shock events are sources of a single compression wave which radiates from the source. The Nature of Sound Waves The diagram above uses the coils of a spring similar to a Slinky toy to represent individual molecules of a sound conducting medium. The molecules in the medium are influenced by adjacent molecules in much the same way that the coils of the spring influence one another. The source of the sound in the model is at the left. The compression generated by the sound source as it moves propagates down the length of the spring as each adjacent coil of the spring pushes against its neighbor. It is important to note that, although the wave travels from one end of the spring to the other, the individual coils remain in their same relative positions, being displaced first one way and then the other as the sound wave passes. As a result, each coil is first part of a compression as it is pushed toward the next coil and then part of a rarefaction as it recedes from the adjacent coil. In much the same way, any point in a sound conducting medium is alternately subjected to compression and then rarefaction. At a point in the area of a compression, the pressure in the medium is positive. At a point in the area of a rarefaction, the pressure in the medium is negative. How does ultrasonics vibrations work? Ultrasonic vibrations are mechanical longitudinal waves that • Achieve deformation in plastic materials • Cause friction between molecules The resulting friction heat generates a melt that bonds the joining partners within the molecules. Friction occurs due to impedances in the material, absorption and reflection of the mechanical vibration: • Internal friction in the molecule bond = dissipative work • External friction between joining materials = surface friction Thermographic illustration of temperature increases during welding Vibration – amplitude – frequency During the ultrasonic welding process mechanical vibrations with defined amplitude, force and duration are applied to the materials to be welded. Due to intermolecular and surface friction heat is generated and melts the material. The core of the ultrasonic welding system is the stack. It is made up of the piezoelectric converter, the booster (amplitude transformer) and the sonotrode. The stack contracts and expands with the ultrasonic frequency. The resulting vibrations are longitudinal waves. The travel of the weld tool, meaning the distance between the peak position and the zero position, is referred to as amplitude - in ultrasonic welding the amplitude is between 5 and 50 μm. Amplitude-frequency- wave length-definition The wave length λ is calculated from the sound velocity, which is a material characteristic, and the frequency. • Definition of amplitude The amplitude a is defined as half the oscillation amplitude, i.e. from zero to peak value. Unit: Micrometer [µm] • Definition of frequency The frequency f is the number of cycles per unit of time Unit: Hertz [Hz] is 1 electrical wave length per second. • Definition of wave length The wave length l (Lambda) is the distance between two equal states along a wave. Schematic diagram of the sonotrode movement What is ultrasonic welding? Ultrasonic welding of thermoplastic materials is a weld technology utilizing mechanical vibrations to generate heat due to molecular friction. These vibrations excite the molecules in the plastic so that they start moving. The plastic becomes soft and starts melting. The components are bonded by cohesive or form-fit joints After a short hold time under pressure, they are firmly joined molecularly. Ultrasonic plastic welding technology Ultrasonic joining technology has been established as joining method for technical thermoplastic materials in a large variety of applications throughout the plastic- processing industry, since 1961. Due to: • high process speeds • repeatable weld results the technology is preferred for high-volume production in the automotive, electrical, medical, packaging, hygiene, and filter industry. Good bonding quality in terms of strength, tightness and visual appearance are particularly achieved if the part material and design is suited for the ultrasonic process. This means that from the beginning they must be designed such that ultrasonic waves are focused in the weld zone. • The factors below control the weld process: Component and ultrasonic weld factors The ultrasonic welding system The complete ultrasonic welding system is composed of active and passive components. The active components generate the vibrations, transfer them, and apply them into the weld parts the passive components absorb the resulting forces, maintain the parts in position, and particularly support the weld joint. The converter, booster, and sonotrode are combined to form what is referred to as stack. The fully digital ultrasonic generator uses the supply voltage to generate high voltage in the respectively required ultrasonic frequency. All data relevant for the weld process are precisely measured and analyzed. The generator protects the vibrating system from overload, keeps the amplitude (tool movement) at a constant level, and compensates for the changing vibrating behavior of different weld tools. The converter represents the interface between the electrical and the mechanical area. Utilizing the inverse piezoelectric effect, it converts the electrical oscillation to mechanical longitudinal vibration and transfers it to the booster or the sonotrode. The amplitude transformer, often referred to as booster, increases or reduces the amplitude coming from the converter. Being suspended at nodal plain, the booster can also be used for vibration-free support of the stack in the ultrasonic welding machine and for transmission of forces. The sonotrode being the actual active weld tool, transmits the mechanical vibrations into the part; i.e. it launches the ultrasonic vibration. Depending on its design geometry, the sonotrode can either increase or reduce the amplitude. The stack is always adapted and optimized with regard to the plastic material and the geometry of the part’s contact. Stack, comprising the converter, booster and sonotrode / horn Ultrasonic plastic welding The secret of ultrasonic plastic welding is focusing the ultrasound with an energy director. In this way, it is possible to generate heat and subsequently melt, restricted to a locally defined area, while using only little energy. Large-area contact surfaces are counterproductive; they require high power and only achieve undefined joining areas with poor strength. Energy focusing is achieved by: • the energy director (ED) • sonotrode / horn design • contouring of the anvil or sonotrode / horn profile • part before and after welding Possibilities of energy focusing due to variations in the joint design / spot welding Possibilities of energy focusing due to variations in sonotrode / horn design / staking Focusing by the sonotrode / horn shape (weld tool); e.g. during the ultrasonic staking process, the sonotrode assumes the task of energy concentration. The centering tip serves as melting initiation aid. Possibilities of energy focusing due to variations in anvil structures or in the sonotrode / horn Focusing by means of contouring of the anvil or in sonotrode / horn structure for web materials such as film, cardboard, and nonwovens. Local deformation is achieved by anvil or sonotrode / horn contours.
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